Glial form and function are extraordinarily divergent with evolution, and human astrocytes are virtually unique in their pleomorphism and fiber complexity. In particular, human astrocytes are larger and more structurally complex than rodent glia, and coordinate the actions of vastly more synapses within their geographic domains. Engrafting neonatal mice with human glial progenitor cells (“hGPCs”) to establish brains chimeric for human astrocytes has permitted assessment of the relative contributions of glial cells to the species-specific aspects of human cognition. These human glial chimeric mice exhibit substantially enhanced activity-dependent plasticity and learning establishing the potential of their use to assess human-specific aspects of the contributions of astrocytes to cognition (Han et al., “Forebrain Engraftment by Human Glial Progenitor Cells Enhances Synaptic Plasticity and Learning in Adult Mice,” Cell Stem Cell 12: 342-353 (2013)).
Astrocytic involvement in human cognitive disorders has never been studied, and yet its role may be profound. As a case in point, a number of conditions, especially several neuropsychiatric disorders, are specific to humans. Yet while human neuronal cytoarchitecture is not very different from that of primates, astrocytic pleomorphism exhibits a quantal leap with human evolution, concurrent with the appearance of a number of neuropsychiatric, neurodevelopmental, and neurodegenerative conditions that appear unique to humans. In particular, glial pathology has been noted to contribute to a broad set of neuropsychiatric and neurodegenerative diseases traditionally considered disorders of solely neuronal dysfunction (Di Giorgio et al., “Human Embryonic Stem Cell-Derived Motor Neurons are Sensitive to the Toxic Effect of Glial Cells Carrying an ALS-Causing Mutation,” Cell Stem Cell 3(6): 637-648 (2008); Di Giorgio et al., “Non-Cell Autonomous Effect of Glia on Motor Neurons in an Embryonic Stem Cell-Based ALS Model,” Nat. Neurosci. 10(5):608-614 (2007); Verkhratsky et al., Astrogliopathology in Neurological, Neurodevelopmental and Psychiatric Disorders,” Neurobiol Dis. pii: S0969-9961(15)00103-5 (2015); Meyer et al., “Direct Conversion of Patient Fibroblasts Demonstrates Non-Cell Autonomous Toxicity of Astrocytes to Motor Neurons in Familial and Sporadic ALS,” Proc Natl Acad Sci USA. 111(2): 829-832 (2014); and Yamanaka et al., “Astrocytes as Determinants of Disease Progression in Inherited Amyotrophic Lateral Sclerosis,” Nat Neurosci. 11(3): 251-253 (2008)).
Huntington's disease (“HD”) is a prototypic neurodegenerative disorder, characterized by abnormally long CAG repeat expansions in the first exon of the Huntingtin gene. The encoded polyglutamine expansions of mutant huntingtin protein disrupt its normal functions and protein-protein interactions, ultimately yielding widespread neuropathology, most rapidly evident in the neostriatum. Yet despite the pronounced loss of medium spiny neurons (“MSNs”) of the neostriatum in HD, and evidence of glial dysfunction (Tong et al., “Astrocyte Kir4.1 Ion Channel Deficits Contribute to Neuronal Dysfunction in Huntington's Disease Model Mice,” Nat. Neurosci. 17:694-703 (2014) and Shin et al., “Expression of Mutant Huntingtin in Glial Cells Contributes to Neuronal Excitotoxicity,” J Cell Biol. 171:1001-1012 (2005)), few studies have investigated the specific contribution of glial pathology either to striatal neuronal dysfunction in HD, or more broadly, to disease phenotype. The lack of understanding of the role of glial pathology in HD has reflected the lack of in vivo models that permit the separate interrogation of glial and neuronal functions in HD, particularly so in humans. Indeed, this gap in knowledge is especially concerning in light of the marked differences between human and rodent glia; human astrocytes are larger and more structurally complex than rodent glia, and coordinate the actions of vastly more synapses within their geographic domains (Oberheim et al., “Uniquely Hominid Features of Adult Human Astrocytes,” J. Neurosci. 29(10): 3276-3287 (2009) and Oberheim et al., “Astrocytic Complexity Distinguishes the Human Brain,” Trends in Neurosci. 29(10): 1-10 (2006)). Accordingly, mice neonatally engrafted with astrocyte-biased hGPCs, which develop brains chimeric for human astroglia and their progenitors (Windrem et al., “A Competitive Advantage by Neonatally Engrafted Human Glial Progenitors Yields Mice whose Brains are Chimeric for Human Glia,” J. Neurosci. 34(48): 16153-16161 (2014)), exhibit substantially enhanced activity-dependent plasticity and learning (Han et al., “Forebrain Engraftment by Human Glial Progenitor Cells Enhances Synaptic Plasticity and Learning in Adult Mice,” Cell Stem Cell 12(3): 342-353 (2013)). Yet this relatively greater role of human astrocytes in neural processing suggests the potential for glial pathology to wreck especial havoc within human neural circuits, with attendant implications for the human neurodegenerative disorders.
The present invention is directed to overcoming these and other deficiencies in the art.